The 3rd Generation Partnership Project, 3GPP, is responsible for the standardization of the Universal Mobile Telecommunication System, UMTS, and Long Term Evolution, LTE. The 3GPP work on LTE is also referred to as Evolved Universal Terrestrial Access Network, E-UTRAN. LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink and is thought of as a next generation mobile communication system relative to UMTS. In order to support high data rates, LTE allows for a system bandwidth of 20 MHz, or up to 100 MHz when carrier aggregation is employed. LTE is also able to operate in different frequency bands and can operate in at least Frequency Division Duplex, FDD, and Time Division Duplex, TDD, modes.
In an UTRAN and an E-UTRAN, a User Equipment, UE, i.e. a wireless device, is wirelessly connected to an access node or Radio Base Station, RBS, commonly referred to as a NodeB, NB, in UMTS, and as an evolved NodeB, eNodeB or eNB, in LTE. A Radio Base Station, RBS, or an access node is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE. In Wireless Local Area Network, WLAN, systems the wireless device is also denoted as a Station, STA.
In the future communication networks, also referred to as the 5th generation mobile networks, there will be evolvement of the current LTE system to the so called 5G system. Due to the scarcity of available spectrum for future mobile, wireless communication systems, spectrum located in very high frequency ranges (compared to the frequencies that have so far been used for wireless communication), such as 10 GHz and above, are planned to be utilized for future mobile communication systems.
For such high frequency spectrum, the atmospheric, penetration and diffraction attenuation properties can be much worse than for lower frequency spectrum. In addition, the receiver antenna aperture, as a metric describing the effective receiver antenna area that collects the electromagnetic energy from an incoming electromagnetic wave, is frequency dependent, i.e., the link budget would be worse for the same link distance even in a free space scenario, if omnidirectional receive and transmit antennas are used. This motivates the usage of beamforming to compensate for the loss of link budget in high frequency spectrum.
Hence, future communications networks are expected to use advanced antenna systems to a large extent. With such antennas, signals will be transmitted in narrow transmission beams to increase signal strength in some directions, and/or to reduce interference in other directions. The beamforming will enable high data rate transmission coverage also to very distant users which would not realistically be covered with normal sector-wide beams, which have lower antenna gain. Beamforming may be used at the transmitter, at the receiver, or both. In a large part of the spectrum planned for 5G deployments the preferred configuration is to use a large antenna array at the access node and a small number of antennas at the wireless device. The large antenna array at the access node enables high-order transmission beamforming in the downlink.
Whenever handover is performed in such a system, for example from one access node to another, or from one frequency band to another, then a good beam direction at the handover target (i.e. the new access node or the new carrier frequency) towards the wireless device needs to be found in order to sustain high data rate transmission. Furthermore, in systems with very high-gain narrow beamforming, even just performing synchronization or exchanging some initial control signaling messages at the handover target may require selection of a sufficiently good beam direction in order for the access node and the wireless device to hear each other sufficiently well.
Beam reception quality metrics may be e.g. some indication of the received power or Signal to Noise and Interference Ratio, SINR. The reporting may be performed either over the already existing connection (with the serving access node or serving frequency band, e.g. using RRC signaling) or signaling over a radio link using the newly found beam at the handover target.
A proposed way to realize this reporting principle is that the wireless device sends a so called Uplink Synchronization Signal, USS, in the uplink towards the access node from which the selected (e.g. best) beam was received (or possibly to another access node or to multiple monitoring access nodes). The USS can indicate the selected beam by the timeslot in which the USS is transmitted (a typical USS sequence may be 1, 2 or 3 OFDM symbols long). To support this mode of reporting a number of timeslots (e.g. with a length of 1, 2 or 3 OFDM symbols each) have been configured, each mapping towards one of the beams in a so called beam sweep. An alternative way of USS based reporting is that there is only one reporting occasion (e.g. timeslot) (possibly per candidate access node), but the symbol sequence used in the USS indicates the selected beam through a preconfigured mapping between USS sequence and beam (e.g. between USS and measured beam reference signal).
A USS may consist e.g. of a symbol sequence that is similar (or equivalent) to a random access preamble, e.g. a Zadoff-Chu sequence, or some other sequence with good autocorrelation and cross-correlation properties. USS based reporting, especially the alternative with a single reporting occasion and USS sequence to beam mapping, is preferable in many handover situations, because it is fast and transmission resource efficient and works even if the wireless device loses its connection with the old serving access node during the handover preparation (i.e. during the beam sweep).
A beam sweep may consist of a variable number of beams depending on the situation. Often, quite many beams may be required, especially when the candidate beams originate from multiple candidate access nodes. However, when the number of beams in the sweep is substantial also this method runs into problems due to that each beam in a sweep has to be mapped to a unique USS sequence, in order for the wireless device to be able to indicate to the network which beam in the sweep that was perceived as the best. That makes USS sequences a scarce resource and a potential limiting factor for the beam sweep, which in turn may hamper the handover performance.